For many years, devices have existed for evaluating parameters of a semiconductor wafer at various stages during fabrication. There is a strong need in the industry to evaluate the parameters of multiple-layer thin film stacks on wafers using non-contact optical metrology tools. In these devices, a probe beam of radiation is directed to reflect off the sample and changes in the reflected probe beam are monitored to evaluate the sample.
One class of prior measurement devices relied on optical interference effects created between the layers on the sample or the layer and the substrate. In these devices, changes in intensity of the reflected probe beam caused by these interference effects are monitored to evaluate the sample. In many applications, the probe beam is generated by a broad band light source and such devices are generally known as spectrophotometers.
In another class of instruments, the change in polarization state of the reflected probe beam is monitored. These devices are known as ellipsometers.
As thin films and thin film stacks have become more numerous and complex, the industry has begun developing composite measurement tools that have multiple measurement modules within a single device. One such tool is offered by the assignee herein under the name Opti-Probe 5240. This device includes a number of measurement modules including a broad band spectiophotometer and a single wavelength, off-axis ellipsometer. The device also includes a broadband rotating compensator ellipsometer as well as a pair of simultaneous multiple angle of incidence measurement modules. The overall structure of this device is described in PCT WO 99/02970, published Jan. 21, 1999, incorporated herein by reference. The Opti-Probe device is capable of deriving information about ultra-thin films and thin film stacks with a high degree of precision.
There is a trend in the semiconductor industry to utilize very thin layers. For example, today, gate dielectrics can have a thickness less than 20 Å. It is anticipated that even thinner layers will be used. There is a need to measure the thickness of these very thin layers with a precision and repeatability to better than 0.1 Å. While the Opti-Probe device is capable of making such measurements with the necessary precision, problems have arisen with respect to repeatability, especially with ultra thin films. Repeatability means that if the same measurement is made at two different times, the same result for layer thickness will be produced.
After considerable investigation, it has been determined that variations in measurements over time are strongly affected by atmospheric conditions such as temperature, humidity and exposure time to the air. For example, the measured layer thickness could be considerably higher when the humidity is relatively high. In addition, the thickness of the layer can be effected by the growth of a contaminant layer, even in so called “clean room” environments. In fact, it is known that a clean room can contain a wide variety of contaminants including plastics, lubricants, solvents, etc. The variation in measurement due solely to atmospheric conditions can be on the order of 0.1 Å which substantially reduces the likelihood of making repeatable measurements with a precision of 0.1 Å. In order to improve the repeatability of the measurements results, it would be desirable to remove the contaminant layer prior to measurement.
There are many types of wafer cleaning procedures used in a semiconductor fabrication facility. However, any cleaning procedures which require contact with the wafer, such as cleaning solutions, would not be desirable at this stage of fabrication since it can damage or contaminate the gate dielectric or the wafer. Additionally, most chemical cleaning processes require a drying cycle during which time a new hydrocarbon contamination layer could reform.
One suitable type of wafer cleaning system is described in our copending application Ser. No. 09/294,869, filed Apr. 20, 1999, and incorporated herein by reference. One embodiment of the system described in the latter patent application includes a microwave generator for exciting water molecules in order to drive off contaminants. Another approach described in the latter application was the use of a radiant heating source to drive off contaminants. Various additional combinations including microwave and radiant excitation along with UV radiation or a stream of frozen carbon dioxide pellets were suggested.
Another wafer cleaning system is described in PCT Application Serial No. WO 99/35677 published Jul. 15, 1999. The device disclosed in this application relies primarily on radiant heating using tungsten halogen quartz lamps. An important aspect of the device in the latter application is the presence of high-energy light wavelengths for breaking bonds in the contaminant layer. The wafer cleaner described in this PCT application has a single chamber. Cooling can be achieved through the use of a water-cooled bottom reflector in the chamber.
After considerable experimentation, it has been determined that the principal mechanism for removing contaminants in the approaches described above relates directly to an increase in the temperature of the wafer. Although microwave excitation and radiant light exposure both function to increase the temperature of the wafer, the latter two approaches are not the most efficient method of raising the temperature of the wafer. Therefore, it is believed that the best approach for preparing a wafer for measurement is to heat the wafer directly, by conduction.
Direct conductive heating has many advantages. For example, direct conductive heating can raise the temperature of the wafer to the desired temperature much faster than with either microwave or radiant energy exposure given the same amount of input energy. In addition, direct conductive heating can produce a more uniform and repeatable temperature rise in the wafer without complex equipment design.
Further experimentation also revealed that optimal results can only be achieved if the process is carefully controlled. Careful control includes heating each wafer to the same temperature, subjecting each wafer to the same cooling cycle and insuring that the time between the end of the cooling cycle and the beginning of the measurement cycle in the metrology tool is the same for all wafers.
Accordingly it is an object of the subject invention to provide an improved wafer-cleaning module which can accurately and repeatably remove contaminants from a wafer prior to measurement in a metrology tool.